A new class of solar energy converters is described, based on photon-enhanced thermionic emission of electrons into a vacuum region between two electrodes. In contrast to conventional thermionic converters, the two electrodes are at the same temperature, and the cathode is side-illuminated rather than front-illuminated. This configuration leads to several advantages: close coupling of multiple units in a series connection high-voltage configuration; higher area for electron emission; and recovery of waste heat at a higher temperature compared to heat recovery from the anode of a conventional thermionic converter. The conversion efficiency from solar radiation to electricity of the new photo-thermionic device may be in the range of 30-40% at moderate operation temperatures and moderate concentration of the incident sunlight. Recovery of the waste heat and further conversion from heat to electricity may raise the overall conversion efficiency to the range of 40-50%. (C) 2013 Elsevier B.V. All rights reserved.